The present invention relates to a method of determining or contrasting material properties of a contact formed between a measurement tip of a microscopic probe and a sample surface of a sample material according to the preamble of appended claim 1, wherein a distance modulation is applied for modulating a distance between a support of the microscopic probe and the sample surface in a direction essentially normal to the sample surface and wherein a normal force signal is measured and demodulated.
The present invention also relates to a device for determining mechanical properties of a contact formed between a measurement tip of a microscopic probe and a sample surface of a sample material according to the preamble of appended claim 16, said device comprising:                said microscopic probe,        a sample stage adapted to hold said sample material,        distance modulating means adapted to modulate a distance between said sample stage and a support of said microscopic probe,        a force sensing means adapted to sense a normal force effective on said microscopic probe from a deformation of said microscopic probe and to provide a normal force signal indicative of said normal force,        a control/analysing unit operatively connected with at least said distance modulating means and adapted to demodulate the normal force signal and to determine said material properties from the demodulated normal force signal.        
The present invention further relates to a control/analysis unit for use in a microscopic measurement device of the above-mentioned type, e.g. an atomic force microscope, and to a computer program product.
With the growing impact of nano-technology, the need for measuring and analysis methods with spatial resolution on the nanometer scale increases, too.
For instance, using Atomic Force Microscopy (AFM) [1] allows imaging of a three-dimensional topography of sample surfaces on micrometer and nanometer scales. Employing sharp measurement tips attached to micro-fabricated probes, so-called cantilevers, which are elastically pressed onto a sample surface, in contact mode of the AFM a direct mechanical sensing of the sample surface is performed on scales ranging from 500×500 micrometers down to scales of several nanometers and atomic scale. Increasingly, it is attempted to determine further sample characteristics in a spatially resolved manner, e.g. optical, electrical and magnetic, as well as mechanical and tribological properties, e.g. friction.
Friction measurement with an AFM has two different interesting aspects. Firstly, it is possible to study the phenomenon of friction and its causes on a microscopic scale. The AFM has the advantage, that the contact between the measurement tip and the sample surface can often be regarded as an isolated point-like single-asperity contact. Secondly, friction measurement with an AFM can be used for characterising materials with high lateral resolution. In this context it may be helpful to use the friction force for contrasting and thus for laterally distinguishing different components of a sample system. In particular contrasting of regions of the sample surface with different chemical termination with a lateral resolution down to the nanometer scale is desirable. For further characterisation it is often desirable, however, to determine local tribological properties of the sample by means of quantitative measurements of the friction force.
Conventionally, lateral force microscopy (LFM) is used for friction measurement, wherein the lateral force signal is recorded during a scanning motion perpendicular to the axis of the microscopic probe (cantilever). However, calibration of the LFM signal often is problematic, thus many publications rely on non-calibrated or qualitative data. Another disadvantage of this method results from the fact that not only friction forces lead to a torsional deformation of the cantilever, but also conservative lateral forces of mostly topographical origin [2]. In order to identify or eliminate those forces, which do not depend on the scanning direction, it is necessary to analyse the LFM signal not only for one scanning direction but also for a scanning motion in the opposite direction. However, such a procedure is problematic because hysteresis effects of the scanner and drift effects can lead to image distortions. A further disadvantage of lateral force microscopy resides in the slow scanning velocity [3].
The last two problems mentioned above have been addressed by incorporating dynamical methods, wherein in addition to said scanning motion a further lateral oscillation of the relative position of the measurement tip with respect to the sample was excited and the LFM signal analysed by means of a Lock-In technique. Such methods are also referred to as Dynamic Scanning Friction Force Microscopy (DSFFM). In this context, the lateral oscillation can be generated by moving the measurement tip [4] or by means of a lateral sample motion [5]. Excitation frequencies are chosen to lie below the resonant frequency of the cantilever in a range of typically a view kHz up to some 10 kHz. An exception is so-called Acoustic Friction Force Microscopy (AFFM), which uses higher frequencies up into the Megahertz range [6]. Said dynamical modi have the advantage of being less influenced by sample topography than conventional LFM. In addition, use of Lock-In technique results in higher signal stability [7]. A disadvantage of methods with a lateral excitation resides in the fact, that most of the AFMs commercially available today, are not equipped for lateral modulation and that an additional piezo-electric transducer has to be used for modulation purposes.
In addition, AFM can be used for a spatially resolved analysis of elastic properties of a contact formed between the measurement tip and a sample surface. An approach for measuring normal contact stiffness is provided by indentation techniques, which rely on a quasi-static variation of the distance between the probe support and the sample by exiting a normal motion of the sample, from which a deformation of the tip-sample contact in the direction normal to the sample surface is derived from the deformation of the cantilever. Besides conventional force modulation microscopy (FMM) [8] as an example for a modulated technique, analysis of force-distance curves in the contact regime and the so-called Pulsed-Force-Mode [9] as a combination of the aforementioned technique with a scanning motion have to be mentioned here. A common disadvantage of indentation techniques arises from the fact, that they are only suited for soft sample systems, such as biological systems or polymers, because the bending force constant of the cantilever should be of the same order of magnitude than the normal stiffness of the contact, hereinafter also referred to as “contact stiffness”. However, if said contact stiffness lies above the bending force constant of the cantilever, the sensitivity for contact stiffness drops substantial due to the fact that deformation of the contact becomes small with respect to deformation of the cantilever.
Owing to a combination of modulation techniques and Lock-In detection, conventional FMM offers the largest reserve with respect to detection sensitivity among the above-described indentation techniques. However, conventional FMM is increasingly sensitive for friction effects on harder samples [10, 11, 12, 13]. Samples with a Young's modulus up to an order of magnitude of several GPa (e.g. polymers) can be analysed, wherein cantilevers with a bending force constant up to an order of magnitude of 100 N/m are required, which approximately corresponds to the maximum bending force constant of commercially available cantilevers [14] (a typical value for the bending force constant of contact mode cantilevers ranging from 0.01 N/m to 1 N/m). An additional disadvantage arises from the fact that high normal forces have to be chosen when using hard cantilevers because of their low detection sensitivity for normal forces [14], which results in an additional increase of the normal contact stiffness and which may result in material destruction and surface modification in particular for easy damageable sample systems. Furthermore, hard cantilevers with bending force constants of the order of the normal contact stiffness are not well suited for sensing surface topography.
A known approach to study elastic properties of harder samples involves high frequency techniques [15], such as Atomic Force Acoustic Microscopy (AFAM) [16], Ultrasonic Force Microscopy [17], and Scanning Local Acceleration Microscopy [18], wherein the modulation frequency corresponds to or lies above the first resonant frequency of the cantilever. However, these techniques share the common disadvantage of requiring additional efforts with respect to the experimental setup, for instance by requiring an additional piezo-electric actuator with suitable driving means as well as suitably fast detectors and electronics. Furthermore, the underlying physical theory is much more complicated and less manageable than in the case of low-frequency methods [14]. In addition, the aforementioned techniques also involve use of cantilevers with hard bending force constants and high normal forces up to the μN range.
A known method providing an approach to elastic sample characteristics with softer cantilevers is known as Magnetic Force Modulation Microscopy (MFMM) [19]. MFMM is a direct force modulation technique, which uses a modulated magnetic field at the free end of the cantilever for generating a magnetic force. To this end, a small magnetic particle is glued to said end of the cantilever. Alternatively, a thin magnetic film can be applied to the cantilever [12]. In contrast to conventional FMM, in the context of this technique the bending force constant of the cantilever does not have to be of the same order of magnitude as the contact stiffness in order to achieve good sensitivity for elastic properties of the tip-sample contact. However, the additional instrumental requirement must be regarded as an inherent disadvantage of said method. Furthermore, the finite magnetic force effectively limits the maximum contact stiffness suitable for investigation, such that only softer sample systems with an Young's modulus up to 10 GPa [12]) can be studied. As with conventional FMM, buckling effects due to friction are not negligible, if the contact stiffness takes on the same order of magnitude or exceeds the buckling force constant [12]. Friction effects of the above-mentioned kind are not to be expected in the context of an extension of DSFFM (Dynamic Scanning Friction Force Microscopy) for measuring lateral contact stiffness by means of a lateral modulation technique [20, 10]. Disadvantages of said method reside in an additional instrumental requirement due to lateral modulation as well as in a difficulty of calibrating the LFM signal sensitivity and modulation amplitude.
Thus, there is a need for a method and a device of the above-mentioned types, which enable comprehensive spatially resolved analyzing of local elastic and tribological properties of a sample surface while obviating the above-mentioned disadvantages of the prior art, and which can be employed to analyse said properties locally at a sample location as well as for providing a spatially resolved mapping of said properties in the form of an imaging method.